Such a method is known from United States Patent document U.S. Pat. No. 4,742,042. In the known method a layer of a mixture consisting of a dye precursor, bis(3-allyl-4-hydroxyphenyl)sulphone as the developer, a heat-sensitive substance (heat sensitizer) and a binder are provided on a support. By locally heating said layer with, for example, a laser a coloured image is formed in said layer.
A disadvantage of the known method is that only the compounds which react with said hydroxyphenyl coupler can be used as dye precursors. According to said Patent document, fluoranes and fluorenes are suitable precursors. The colour of the dyes formed ranges from blue to green. To obtain a mixed colour a second leuco dye must be added to the mixture. In said United States Patent document no description is given of how other colours, such as violet, yellow and orange must be made. A further disadvantage is that said dyes can only be formed in a thermal process, i.e. using infrared radiation, and not in a photochemical process using ultraviolet radiation. A still further disadvantage is that the layer in which the dye is formed must be provided with a protective coating comprising UV-absorbing substances to improve the resistance of the colour image formed against light and other environmental factors.